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Filling the world with light

How did you start working with blue LEDs?

In the 1980s many people were working on creating blue LEDs using zinc selenide. I was then working at Nichia Corporation and also working towards a PhD. At the time, it was possible to obtain a PhD by just publishing papers, which is called a “paper degree”. So I was mostly interested in publishing as many papers as possible to get my PhD. I began to work on gallium nitride as not many people were working on that material.

Was it exciting when you found you could make an efficient blue LED?

A blue LED is made from various layers of gallium nitride, but you also need indium gallium nitride, which is the key material as it emits blue and green light. Yet when we make LEDs with indium gallium nitride there is a huge dislocation density – a measure of deformation in the material – of around 10⁹ cm^–2. Given this huge number, many expected that the material would never work as LEDs because you generally needed a dislocation density less than 10³ cm^–2 to produce an efficient LED. But we managed to get it to work. Given the high dislocation density, nobody knows the physics behind why it works and is so efficient – it’s still a mystery.

And the Nobel Foundation finally recognized the breakthrough last year?

Yes, I shared the 2014 prize with Isamu Akasaki and Hiroshi Amano. Although it’s funny that indium gallium nitride – the key to blue LEDs – wasn’t mentioned by the Nobel Foundation in the prize announcement. They ignored it. It’s incredible as you can’t make a blue LED without it.

Given the 20-year gap, did it come as surprise to finally win?

No, because ever since the breakthrough in 1993, the Japanese media have followed me in October every year saying that I should be awarded a Nobel prize.

How did you find out you had won?

When the Nobel Foundation called me. It was 2 a.m. local time in California, so I was sleeping.

Has your life changed since?

Not by much. The only difference is that the Japanese media follow me more now and students recognize me. The University of California, Santa Barbara, has also given me a free parking space and I don’t have to teach anymore.

How will you spend the money?

The amount of money you get for winning a Nobel prize is smaller these days. The three of us got $400,000 each. For Nobel week – a series of events marking the award, including the Nobel banquet – I paid the expenses of 14 guests, which came to about $100,000. I was then left with $300,000 and then $150,000 after US taxes. I have also donated half – $75,000 – to the University of Tokushima.

What will they use that money for?

I didn’t tell them how to spend it, so they can use it however they see fit.

How are relations now with Nichia?

I left Nichia in 1999 to move to the University of California, Santa Barbara. As I did not sign a non-disclosure agreement, the firm then filed a lawsuit for infringement of trade secrets. I countersued them using Japanese patent law, which states that an invention belongs to the inventor and not the company. In 2005 I settled with Nichia for a one-off payment of $8m. Since winning the prize I have tried to improve relations with Nichia, but they are not interested. So I won’t be trying again.

Do you feel disappointed by this?

I expected that they would say no, so I am not disappointed. It was actually my former adviser at the University of Tokushima – Osamu Tada – who said that I should aim to improve relations with them.

As your prize is deeply connected with light, will you be involved with the International Year of Light?

I have a lot of invitations to many events around the world, but at the moment I am very busy.

What are you working on now?

Together with my colleagues, we are working on next-generation lighting – laser lighting. Using laser diodes we can obtain a luminescence 1000 times higher than an LED, so we can make a very bright light source. But it will take time to achieve this, maybe 5–10 years.

What are the challenges?

Currently, laser diodes are very expensive – around $10 for a laser diode, but just $0.10 for an LED. Also, the “wall plug” efficiency of laser diodes is around 30%, this is not high enough compared to the blue LED, which is about 50–60%. So we have to find ways to reduce the cost and improve the efficiency further, but we are very excited about the prospect of laser diodes.

Between the lines

A doodle with spaces coloured in yellow, pink and different shades of green — Squiggle maths: A new book shows that any doodle you draw obeys the same mathematical formula, among other fun maths puzzles and challenges. (CC-BY Dan Zen)

Mathematical doodling

The next time you find yourself idly sketching loops and curves on a notepad, Tim Chartier has a request: he wants you to turn your doodles into mathematics. A mathematician at Davidson College in North Carolina, US, Chartier is the author of Math Bytes, a grab-bag of a book that is full of quirky everyday applications of mathematics, including the aforementioned doodles. It turns out that any squiggle you create will obey the formula V + F – E = 2, where V is the number of vertices (the points at which lines intersect each other), F is the number of enclosed areas or faces (basically regions you could colour in) and E is the number of edges (the line segments between the vertices). The relationship between these numbers is called the Euler characteristic after the 18th century mathematician Leonhard Euler, who proved that it holds for all doodles. It has a number of applications, including as a test to determine whether a maze is solvable. Euler’s characteristic is also linked to the famous “travelling salesman problem”, which asks which of many possible paths one should take between a set of vertices in order to minimize the distance travelled along the edges. Conceptually speaking, this isn’t a hard problem to understand, but as Chartier explains, it is actually one of the most challenging puzzles in all of computational mathematics. Most of the chapters in Math Bytes likewise begin with simple set-ups and gradually move on to more complex ideas, and this (plus the large number of hands-on examples) make it a handy guide for anyone involved in science or mathematics outreach.

2014 Princeton University Press £16.95/$24.95hb 152pp

Exploring spirit

Here’s a back-of-the-envelope problem for the rocket scientists reading this: how much conventional rocket fuel would it take to deliver a Space-Shuttle-sized payload from Earth to the Alpha Centauri system in less than 1000 years? The answer – and many other interesting facts concerning the past, present and future of space exploration – can be found in Beyond, a new book written by Chris Impey. An astronomer and popular-science author, Impey makes an affable and generally even-handed guide to this fascinating subject, balancing rhetoric about humankind’s restless curiosity with sober assessments of what is and is not possible. The book refers, vividly, to “the tyranny of the rocket equation”, while the immense cost and long time period required to terraform Mars or send humans to another star are laid out with a fine, clear blend of optimism and realism. The one sour note in Beyond concerns its treatment of “new space” entrepreneurs such as Burt Rutan and Richard Branson. The book was already in press in October 2014, when one of Rutan’s pilots, Michael Alsbury, was killed during a test flight for Branson’s Virgin Galactic project, so it is not Impey’s fault that this setback is barely mentioned. Even so, in light of Alsbury’s death and the still-incomplete investigation into its causes, readers may find it a trifle jarring to read fawning descriptions of Branson’s buccaneering attitude and Rutan’s “entrepreneur’s impatience with red tape”. (After all, what are safety regulations but a form of red tape?) Once Impey finishes gushing about the “breathtaking passion” of these charismatic figures, though, his overall assessment of “new space” is fair-minded and insightful. Far from being moribund, today’s space industry, Impey argues, “may now be where the Internet was in 1995, ready to soar”. Now isn’t that an exciting thought?

2015 W W Norton £16.99/$27.95hb 336pp

Astrofacts at your fingertips

What’s the reddest object in our solar system? Surprisingly, the answer isn’t Mars but a denizen of the Oort cloud called Sedna. This icy, rocky object resembles Pluto except for the high proportion of carbon-based chemicals mixed into its crust, which account for its reddish hue. Sedna’s other claim to fame is that it is currently the most distant object we know of in the solar system: located well outside the orbits of Uranus and Neptune, it orbits the Sun on a highly elliptical path that will eventually take it out to a point that is 937 times farther from the Sun than the Earth is now. As such, Sedna’s reddish glow isn’t going to be readily apparent to backyard stargazers, so it’s not immediately obvious why it deserves its own entry in the pages of Astronomy in Minutes, a pint-sized guide that promises to explain the night sky “in an instant”. Look more closely, though, and you will see that the book’s small size belies its scope. Written by the astronomer and science communicator Giles Sparrow, Astronomy in Minutes includes brief summaries of astrophysical topics such as variable stars, black holes and the H-R diagram of stellar evolution as well as practical stargazing tips. With individual descriptions of what to look for in 60 different constellations, plus some more complex material to whet the appetites of young enthusiasts, it’s definitely worth tucking into a backpack the next time you head out for a bit of stargazing.

2015 Quercus £8.99pb 416pp

Littered with errors

By Michael Banks

Photograph of Swheat Scoop cat litter (CC-BY-SA Ryan Forsythe) Cat litter and radioactive waste – not a combination you would normally expect to come across (although some cat owners may disagree).

But a report by the US Department of Energy has squarely blamed kitty litter for the explosion of a single drum of nuclear waste – dubbed “68660” – that burst open at the Waste Isolation Pilot Plant (WIPP) in New Mexico in February 2014.

A year-long investigation by a nine-member panel – led by David Wilson of the Savannah River National Laboratory – has concluded that the incident was caused by the use of the wrong brand of feline litter.

As cat litter is highly absorbent, for years it has been used to help keep nuclear waste contained. Indeed, each barrel of waste at the WIPP is filled with about 26 kg of the stuff.

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Five amazing physics demonstrations

Even if you’re a hardcore theoretical physicist, I’m sure you’ll agree that experiments are the lifeblood of physics. After all, theory and experiment go hand in hand – and there’s nothing to beat getting your hands dirty to get a proper understanding of the subject.

But how can pupils and students get excited about experiments? Making practical work a key part of exam syllabuses is surely important – yet the danger then is experimental work becomes a chore not a charm.

If you need inspiration, check out the April issue of Physics World magazine, which is now out in print and digital formats. It contains a great feature by Neil Downie – head of sensors at Air Products, Basingstoke, Hampshire, UK and a Royal Academy of Engineering visiting professor at the University of Surrey.

Having run “Saturday science clubs” for children for more than two decades, Downie presents in the feature his five best physics demonstrations of all time. He’s also teamed up with Physics World to create a set of five short films, each showing one of the demos in action.

Apart from being simple and quick to carry out, the beauty of the five demos is that while you don’t need much equipment to get started, you can take the projects in lots of different directions.

If you’re a member of the Institute of Physics (IOP), you can get immediate access to the feature and all five videos with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full digital access to Physics World.

We’ve also included the first of Downie’s films above to whet your appetite, in which he shows the principles of the “Vacuum Bazooka”.

For the record, here’s a rundown of what else is in the issue.

• UK and Italy vie over telescope HQ – An Italian bid to host the headquarters of the world’s largest radio telescope has been judged superior to a British proposal – yet it has failed to get the green light. Edwin Cartlidge reports

• Filling the world with light – After sharing the 2014 Nobel Prize for Physics for the discovery of efficient blue light-emitting diodes (LEDs), Shuji Nakamura talks to Michael Banks about what comes next

• Fostering talent – Science can only blossom if young researchers are rewarded for their growth rather than their “academic ancestry”, says Abraham Loeb

• Fight over light – Robert P Crease explains the fascination with Goethe’s flawed book Theory of Colours, which savagely attacked Newton’s book Opticks

• Communicating discovery – So you’ve made a scientific discovery – what next? Communicating your finding to the public without succumbing to the many pitfalls along the way is a tricky business, as Simon Perks reports

• The will of the WISPs – While many physicists hunt dark matter in the form of WIMPs – “weakly interacting massive particles” – others have turned their attention to a lighter prey: “weakly interacting slim particles”, or WISPs. Edwin Cartlidge reports

• Five amazing physics demonstrations – There’s nothing better than a good physics demonstration to illustrate the subject’s fundamental principles. Neil Downie, who has run Saturday science clubs for children for more than two decades, presents his five best demos of all time

• Bringing the quantum to life – Marta Varela reviews Life on the Edge: the Coming of Age of Quantum Biology by Jim Al-Khalili and Johnjoe McFadden

• Albert and Erwin: decline and fall – Denis Weaire reviews Einstein’s Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics by Paul Halpern

• Freeze brain – The challenge of designing the equipment that makes low-temperature
science possible has kept Roger Mitchell fascinated throughout his 30-year career at the UK firm Cryogenic, where he is now technical director

• Once a physicist – John Harte, an ecologist in the Energy and Resources Group at the University of California, Berkeley, US

• Physics on a coffee break – Karen Yates from Imperial College London, muses over the importance of coffee in physics

Amazing science demo one: The Vacuum Bazooka

This video is the first in a series of “five amazing physics demonstrations” presented by demo aficionado Neil Downie. In addition to his day job in industrial science, Downie has run Saturday science clubs for children for more than two decades, during which he creates fun and innovative science demonstrations that are all simple and quick to carry out.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. In the article, Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how to present each demonstration. Full details of how to access the print and digital issues are available at the bottom of this article.

The first demo is “The Vacuum Bazooka”, in which a vacuum cleaner is transformed into a missile launcher that can propel objects across a room. The video shows the front cover of Physics World being used for target practice, with Downie explaining how the experiment illustrates physics principles including air pressure, acceleration, ballistics, inertia and air flow down tubes.

The Vacuum Bazooka

So what’s this all about? This project redeploys the humble vacuum cleaner as a projectile launcher. Over the years I have made hundreds of Vacuum Bazookas, and thousands more have been made around the world by others. Some people have even used professional laboratory vacuum pumps and managed extraordinary feats such as firing ping-pong balls so fast that they will punch holes in beer cans.

What bits and pieces do I need? The Vacuum Bazooka can be swiftly plugged together from plumbing parts. In my favourite version, it consists of a long plastic tube, at one end of which you connect another long tube at right angles using a plastic T-piece. You’ll also need a suitable projectile, such as a cylinder of wood or a champagne cork, that can pass easily down the main tube, through the T-piece and out the end. Finally, you’ll need a reasonably powerful domestic vacuum cleaner (I use a Henry) that can reduce the pressure in the tube from atmospheric pressure (100 kPa) to 80 kPa or less.

How do I get going? Wearing safety goggles, set up your kit about 5–10 m from a target, such as a table placed on its side. Connect your vacuum cleaner to the end of the tube that’s at right angles to the first. Turn the cleaner on and then let a decent vacuum build up in the tube by placing a sheet of paper over the free end of the T-piece through which the projectile will leave. Reach over and insert your projectile into the far end of the tube, holding it tightly while you point the tube at your target. Wait for a moment and then let go of the projectile. Whoosh! It’ll zoom down the tube, fly out through the T-piece and whack into your target.

And what physics will I learn? The experiment reminds us that we live at the bottom of an “ocean” of air that creates an apparently preposterous pressure on our bodies. By removing the air from the tube, you can accelerate objects really fast because of the resulting pressure differential. Without the sheet of paper, however, the differential would be much smaller and the projectile would move only slowly – and probably jam the T-piece. In case you’re wondering, I’ve estimated that the cork will fly at a speed of (2P₀L/zρ)^1/2, where P₀ is the pressure in the tube, L is its length, z is how far the projectile travels and ρ is the density of air. Why don’t you try checking the equation – perhaps by doing the experiment outside on a lawn? As for the sheet of paper, it rarely gets damaged – the puff of air just before the projectile exits the tube usually blows the paper harmlessly aside.

If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine

Graphene sandwich squares away ice

Sandwiching water between two sheets of the wonder-material graphene causes it to freeze at room temperature in the form of 2D square crystals of ice, rather than its normal hexagonal crystal lattice, according to a team of researchers from the UK, Germany and China. This phase of ice could exist within some other nanostructures, such as carbon nanotubes, and could help explain why water moves unusually in these materials – a result that could have implications for developing more efficient filtration, desalination and distillation technologies.

Water exists in many forms, including liquid, vapour and as many as 15 crystal structures of ice. The hexagonal structure is responsible for the shape of snowflakes and the intricate patterns that form on surfaces when they freeze. “Less noticeable, but equally ubiquitous, is the water found at interfaces and confined in microscopic pores,” says Irina Grigorieva of the University of Manchester in the UK. “In fact, monolayers of water cover every surface around us, even in the driest deserts, and fill in every single microscopic crack on Earth – for example those present in rocks. However, we know very little about the structure and behaviour of such microscopic water – especially when it is hidden from view in capillaries deep inside a material.”

The newly discovered ice films, which are less than 1 nm thick, have a completely different symmetry to that of normal ice – which has a hexagonal structure. “The new phase of ice forms at room temperature, well above the ‘normal’ freezing temperature of water,” explain team leaders Grigorieva and Andre Geim, who is also based at Manchester and is one half of the duo that won the Nobel prize for the discovery of graphene in 2010. “Apart from finding this new phase – not something that happens every day – our result will allow us to better understand the counterintuitive behaviour of water inside nanochannels, such as ultrafast permeation though graphene-oxide membranes.”

Under pressure

The researchers employed high-magnification electron microscopy to look at the atomic structure of water trapped inside a transparent nanoscale capillary made from two sheets of graphene. Graphene is a layer of carbon atoms just one atom thick, and so does not obscure electron-microscopy imaging. “Thanks to strong adhesion between the graphene sheets that form the nanocapillary, pressures inside it can reach as high as 1 GPa, which appears to be an important factor in making water crystallize into ice,” explains Grigorieva.

“To our surprise, we found small square crystals of ice at room temperature, provided the graphene capillaries were narrow enough – that is, those that allow for no more than three molecular layers of water,” she says. “The water molecules form a square lattice, sitting along evenly spaced neat rows running perpendicular to each other.” Such a flat square arrangement is completely uncharacteristic for bulk ice, in which the molecules always form small pyramidal structures, she adds.

Uncharacteristic shapes

Alan Soper of the UK’s Rutherford Appleton Laboratory in Harwell, Oxford, agrees. “Water trapped between graphene sheets under these conditions is likely to crystallize, even at room temperature,” he writes in a Nature News & Views article. “But the fact that it forms a square structure is unexpected.”

The team says that it also attempted to find out how common square ice might actually be in nature using computer simulations. “Our results shows that if the layer of water is thin enough, it should form square ice independently of the exact chemical make-up of the confining nanopore walls,” says Grigorieva. “It is thus likely that square ice is very common on the molecular scale and present at the tip of every microscopic crack or pore in all materials.”

The new observations might be important for understanding how water moves through natural and man-made nanoscale channels. These include aquaporin (a widely occurring channel protein that regulates water flow across cell membranes) and carbon nanotubes.

The research is published in Nature.

This article first appeared on nanotechweb.org

Entangled photons cast a new light on cause and effect

The idea that correlation does not imply causation is well known to scientists and statisticians, but now physicists in Canada have shown that it is not always the case in the weird world of quantum mechanics.

Research in medicine, economics and many other disciplines often relies on showing a statistical correlation between two variables. It is often not clear, however, whether a change in one variable actually causes a shift in the other or whether the two variables are related via a third unmeasured factor. In a drug trial, for example, a higher recovery rate among those who take a certain drug compared with those who choose not to take the drug could be related to a third factor that is linked causally to both – perhaps those who choose not to take the drug are less ill than the others. The answer is to carry out randomized drug trials, in which drugs and placebos are distributed randomly. This means that one variable – whether or not a patient chooses to take the drug – is controlled, rather than being left alone.

In the latest work, a team led by Kevin Resch of the University of Waterloo in Canada and Robert Spekkens of the Perimeter Institute for Theoretical Physics, also in Waterloo, has discovered that in quantum mechanics it is possible to find out whether or not two variables are linked causally without having to control one of the variables. Both variables can in fact be left free, with causation established purely by studying the pattern of correlations that emerge from repeated trials of the quantum system.

Entangled partners

Spekkens and two other theorists devised a scheme in which they start by preparing two photons in an entangled state. They measure the polarization of one of these photons, called A, and then send it and its entangled partner through a gate. The photon that emerges from this gate – denoted B – is in some cases just a transformed version of A, whereas in other cases it is A’s entangled partner. In the first instance, A causes B, while in the second case the two particles are related to one another by a common cause.

Crucially, these two possibilities are combined using a random-number generator, so that for a certain fraction of the time the apparatus establishes a direct causal link between A and B, whereas the rest of the time A and B are two halves of an entangled pair. In other words, the person operating the experiment doesn’t know whether they are dealing with a single particle at two different times or with two entangled particles.

Resch and two of his students put this scheme into practice using polarizers, making one of three measurements on A – distinguishing horizontal from vertical polarization, diagonal from anti-diagonal polarization, or left-circular from right-circular polarization – and then carrying out exactly the same measurement on B. They repeated this process thousands of times to build up reliable statistics, with around a third of the runs dedicated to each measurement type.

Matching polarizations

The researchers established that if A causes B then the result of the polarization measurement at B may either match that at A for all three kinds of polarizers or match it for just one kind. Conversely, if A and B have a common cause, then B’s result will either match A’s for two of the three polarizers or match it for none of them (i.e. measured polarizations will always be different). These correlations were seen in the distribution of results from thousands of runs, with the exact form of that distribution dependent on the “bias” that the researchers chose to apply to the random-number generator – in other words, just how likely the generator was on any given run to emit a transformed version of A rather than spit out A’s entangled partner.

In this way, the team was able to establish a pattern of correlations that could in future be used to identify the unknown bias of a black-box experiment. Their results bear some similarity to those obtained in unpublished work carried out in 2013 by a team led by Vlatko Vedral of the National University of Singapore and Oxford University. Vedral’s group had discovered correlations that imply a direct causal link between quantum states – in their case, between nuclear spins. What Resch and colleagues have done is to find a set of correlations that can prove either the existence of a causal link or that of common causality. “We found out about the earlier work when we were halfway through writing our paper,” says Katja Ried at the University of Waterloo. “But our work is more general. It is more symmetrical.”

Superposition of causation states

Caslav Brukner of the University of Vienna and the Institute of Quantum Optics and Quantum Information in Austria praises the “important and interesting” work, and raises the intriguing prospect that correlations could exist in a superposition of being both causally related and correlated through a common cause. He explains that such superpositions would be analogous to those of position or momentum in quantum mechanics. “If such structures exist in nature,” he says, “the new research might turn out to be useful to detect them.”

The researchers say that their work could help to better understand the extent to which quantum uncertainty is a feature of the real world, as opposed to a limitation in our knowledge of the world. They also believe their results could prove useful in testing components for quantum computers, by helping to distinguish between a component’s input causing its output and a source of external interference acting on both.

The research is published in Nature Physics.

Florida’s declining Space Coast, naming mountains on Pluto and silly rock bands

Artist's impression of Pluto — Name game: does that crater look like a Steve, or maybe a Carol? (Courtesy: IAU/L Calçada)

When I was a young lad back in the late 1960s, my family would join the annual March migration of Canadians to Florida. Along with alligator farms and the endless beaches, the Kennedy Space Center was a popular tourist destination and I can still remember visiting it and getting a solar spinner globe as a souvenir. Sadly, since the end of the Space Shuttle programme in 2011, Florida’s “Space Coast” has fallen on hard times. While there are still rocket launches – there are two planned for April – thousands of NASA employees have been let go and the surrounding communities look worse for wear. The New York-based photographer Rob Stephenson has put together a collection of images taken in and around the centre that he calls “Myths of the Near Future”. To me the photographs evoke the allure of the space age as well as the inevitable decline of any human endeavour.

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‘Twisted light’ gives quantum cryptography a boost

The efficiency of quantum-cryptographic systems could be improved thanks to a new technique that uses “twisted light” to increase the amount of information carried per photon. Developed by an international team of researchers, the technique involves encoding 2.05 bits per photon by using the orbital angular momentum (OAM) of light instead of the more commonly used polarization of light, which only allows one bit per photon. The researchers say their new approach could be extended to achieve 4.17 bits per photon, and it could be used to make cryptographic systems more resilient to external eavesdroppers.

Unlike most other quantum technologies, quantum-cryptographic systems are already being used commercially by some banks and other organizations to ensure the secrecy of their communications. Such systems employ quantum-key distribution (QKD), which allows two parties – say Alice and Bob – to exchange an encryption key secure in the knowledge that it will not have been read by an eavesdropper (say, Eve). This guarantee is possible because the key is transmitted in terms of quantum bits (qubits) of information, which would be irreversibly changed if they were somehow intercepted and read, thereby revealing Eve.

One to two

Most current QKD schemes make use of the different polarization orientations of a photon – vertical, horizontal, diagonal, antidiagonal – but this only allows one qubit to be transferred per photon. In the new work, Mohammad Mirhosseini of the University of Rochester in the US and colleagues worldwide have doubled the amount to 2.05 qubits per photon by encoded their qubits using the OAM and the azimuthal angular position (ANG) of photons. OAM involves the wavefront of a beam of light spiralling around its propagation axis and is sometimes referred to as “twisted light”.

These two properties provide what are known as “mutually unbiased bases” – an essential requirement for QKD. Using such bases means that a correct key is revealed only if Alice encodes the information using a particular basis and Bob measures in that same basis. As the OAM and ANG are mutually unbiased with respect to one another, an eavesdropper would not be able to detect a photon simultaneously in both bases, thereby boosting its security.

Twisted alphabet

Once Alice and Bob have generated their QKD, they publicly announce the basis they have used for each symbol in the key and compare what basis was used for sending and which basis was used for receiving. They only keep the part of the key in which they have used the same bases and this ultimately produces a secure key, which can then encrypt messages and indeed transmit them with regular encryption without the need for quantum cryptography.

Mirhosseini, who is part of Robert Boyd’s group at Rochester’s Institute of Optics, says that the team was able to encode a 7D “alphabet” – seven letters or symbols – using OAM and the ANG. “Our experiment shows that it is possible to use ‘twisted light’ for QKD and that it doubles the capacity compared with using polarization,” he says, further explaining that “unlike with polarization, where it is impossible to encode more than one bit per photon, twisted light could make it possible to encode several bits, and every extra bit of information encoded in a photon means fewer photons to generate and measure”.

The team demonstrated that its system can generate and detect information with 93% accuracy and at a rate of 4 kHz. In the future, the researchers hope to push the rate to the gigahertz level, which is desirable for telecommunication applications. In an earlier experiment that used a strong laser beam instead of single photons, Boyd’s team was able to measure up to 25 modes or bases of OAM and ANG, rather than seven. If that method is applied to the new scheme, it could be used to transmit and measure 4.17 bits per photon using more sophisticated equipment.

The work is published in New Journal of Physics.

Acoustic topological insulator could hide submarines

Sound scatters from most surfaces and creates echoes that can be distracting to listeners and dangerous if you happen to be in a submarine trying to evade detection. Now, researchers have proposed a new “acoustic topological insulator” that could help alleviate such problems by transmitting sound in certain directions without any backscattering. If the material can be built in the lab, it could herald the development of new acoustic devices that have a range of medical and military uses including improving hearing aids and making objects invisible to sonar.

Topological insulators are materials that do not conduct electricity through their bulk volume but are very good conductors on their surfaces. This is possible because of the existence of special “edge states” in which electrons cannot backscatter for topological reasons. Recently, several research groups have been looking at how an acoustic version of a topological insulator – whereby sound waves will travel on the surface of a material but not through its bulk – could be made from a periodic acoustic medium called a “phononic crystal”.

Spinning air

Now, a team led by Baile Zhang at Nanyang Technological University in Singapore has unveiled a new design for an acoustic topological insulator made from a regular array of spinning cylinders.

The design is based on a triangular lattice with unit cells 20 cm in size. Each unit cell contains a rigid solid cylinder at its centre that is spinning at 400 revolutions per second. Each cylinder is surrounded by a concentric shell that is transparent to sound. The rotation of the cylinders causes the air in each shell to rotate, while the remainder of the lattice is filled with stationary air.

Calculations done by the team suggest that sound waves at frequencies between 914–1029 Hz will be guided around the edges of the lattice. Furthermore, the waves move with ease across any defects, disorders, sharp corners or protrusions on the edges of the lattice. This is the same behaviour seen in the electric conductivity on the surface of a topological insulator.

“This structure can guide acoustic waves around its surfaces smoothly without reflection, even in the presence of defects or disorders,” explains Zhang.

One-way travel

Another important feature of these “acoustic edge states” is that sound will only propagate in one direction. This direction depends on whether the cylinders are rotating clockwise or anticlockwise, and therefore can be reversed by altering the rotation. The calculations also suggest that such lattices could be tuned to offer this unidirectional, reflection-free propagation across a range of audible and ultrasonic frequencies.

Zhang told physicsworld.com that the phononic crystals could be used to improve hearing aids by creating systems that are very efficient at channelling sound through the ear canal. He also believes that the technology could be used to create acoustic “invisibility cloaks” that would guide sonar sound waves around the surface of objects such as submarines, thereby hiding them from detection.

“This work constitutes credible proof of the principle of acoustic insulators,” says Thomas Brunet of the University of Bordeaux in France who was not involved in the study. José Sánchez-Dehesa of the Polytechnic University of Valencia in Spain adds that “The challenge now is making this theoretical proposal feasible in a simple and cheap manner.”

This research is described in Physical Review Letters.

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Filling the world with light

How did you start working with blue LEDs?

Was it exciting when you found you could make an efficient blue LED?

And the Nobel Foundation finally recognized the breakthrough last year?